Bug#17867117 - ERROR RESULT WHEN "COUNT + DISTINCT + CASE WHEN" NEED MERGE_WALK 

Problem:
COUNT DISTINCT gives incorrect result when it uses a Unique
Tree and its last inserted record has null value.

Here is how COUNT DISTINCT is processed, given that this query is not
using loose index scan.

When a row is produced as a result of joining tables (there is only
one table here), we store the SELECTed value in a Unique tree. This
allows elimination of any duplicates, and thus implements DISTINCT.

When we have processed all rows like this, we walk the Unique tree,
counting its elements, in Aggregator_distinct::endup() (tree->walk());
for each element we call Item_sum_count::add(). Such function wants to
ignore any NULL value, for that it checks item_sum -> args[0] ->
null_value. It is a mistake: when walking the Unique tree, the value
to be aggregated is not item_sum ->args[0] but rather table ->
field[0].

Solution:
instead of item_sum -> args[0] -> null_value, use arg_is_null(), which
knows where to look (like in fix for bug 57932).

As a consequence of this solution, we have to make arg_is_null() a
little more general:
1) Because it was so far only used for AVG() (which always has a
single argument), this function was looking at a single argument; now
that it has to work with COUNT(DISTINCT expression1,expression2), it
must look at all arguments.
2) Because we start using arg_is_null () for COUNT(DISTINCT), i.e. in
Item_sum_count::add (), it implies that we are also using it for
COUNT(no DISTINCT) (same add ()). For COUNT(no DISTINCT), the
nullness to check is that of item_sum -> args[0]. But the null_value
of such item is reliable only if val_*() has been called on it. So far
arg_is_null() was always used after a call to arg_val*(), so could
rely on null_value; but for COUNT, there is no call to arg_val*(), so
arg_is_null() has to call is_null() instead.

Testcase for 16539979 by Neeraj. Testcase for 17867117 contributed by
Xiaobin Lin from Taobao.